Problem with applying the Subdivision modifier
It's a bad idea to apply subdiv. If you animate a mesh with an armature, try to keep the topology clean and sufficient, but not redundant. Add 3 loops where the limbs are to allow the mesh stretch correctly. When you skin your mesh to the armature (weight paint), it's much easier to handle non subdivided mesh. And keep the armature modifier before the subdivision.
Yes. maybe your model has insufficient geometry, but it doesn't mean you have to subdiv the entire mesh. Just think where and how you want the model to bend, and add geometry (by hand) just there. And master weight paint mode, it will change your model skinning drastically.
As a good start I'd recommend some tutorials to get understand the basics. I like the rigging series by DanPro on youtube. Try this one, to see what matters on rigging. There are tons of nice tutorials, just find that will suit your skills and taste.